Method and apparatus for controlling wastewater treatment processes

ABSTRACT

Methods and apparatus for continuing, automated control of wastewater treatment processes. In certain preferred embodiments, method and apparatus for control of aeration in suspended growth biological treatment processes, especially in activated sludge processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

Benefit of international priority and internal priority is claimed underall applicable international treaties and national laws throughout theworld as to the subject matter of U.S. Provisional Patent ApplicationSer. Nos. 60/412,817 and 60/479,150, respectively filed on Sep. 24, 2002and Jun. 18, 2003 in the names of David T. Redmon, Thomas E. Jenkins,Ian Trillo Fox, Juan De Dios Trillo Monsoriu and Timothy D. Hilgart, andthis application is also a continuation of U.S. Ser. No. 10/667,893,filed Sep. 23, 2003, now U.S. Pat. No. 7,449,113, and entitledCONTROLLING WASTEWATER TREATMENT PROCESSES, incorporated herein byreference.

TECHNICAL FIELD

This invention relates to methods and apparatus for continuing,automated control of biological wastewater treatment processes. Incertain preferred embodiments, it relates to control of aeration insuspended growth biological treatment processes, especially in activatedsludge processes.

BACKGROUND OF THE INVENTION

Most forms of biological processes for treatment of wastewater involveintroducing oxygen-containing gas into wastewater with some form ofenergy-consuming apparatus. Generally, an electric motor is the energyconsumer, and it powers some kind of agitator, compressor or blower thatprovides driving force to distribute the oxygen-containing gas in one ormore tanks containing wastewater. For many years it has been apparentthat the cost of electricity to operate such equipment is one of thelargest, and often the largest, operational cost of wastewater treatmentplants.

In the early history of the art of biological treatment, process controlwas “manual”. Aided to an inadequate extent by visual observation and byinstrumentation that was usually limited and rudimental, plant operatingpersonnel adjusted gas flow in an attempt to match that flow to theamount of oxygen consumed in the biological process. Too much flow,overshoot, wasted electricity. Too little, undershoot, impaired thequality of treatment.

As the art progressed, it was recognized that savings in electricity andmore consistent quality of treatment could be achieved with better andmore complete instrumentation. Then, it began to be apparent that majorgains in energy savings and quality could be attained through automaticcontrol of gas flows and other aspects of the processes.

Since at least as early as the 1960s, efforts at automated control ofthe flow of oxygen-containing gas into biological wastewater treatmentprocesses have included measurements of the DO (dissolved oxygen) levelin the wastewater in the treatment tank. Gas flow is automaticallyreduced if DO exceeds a predetermined target or set point and increasedif DO falls below the target. This mode of operation reduced but did noteliminate the problem of overshoot and undershoot of the true oxygen andenergy requirements of the biological processes.

Since as least as early as the 1970s, the need to conserve energy andtightening regulations on plant effluent quality have provided ample andcontinuing motivation to develop better forms of automated control.However, despite many suggestions for additional and/or other modes ofautomatic control, in actual practice, control based primarily on DOlevels, with ensuing energy wastage and quality challenges, has remainedquite popular.

Present-day continuation of the popularity of control based primarily onDO measurements, accompanied by wastefulness and quality problems,suggests there is a long-felt, unsatisfied need for improvements incontrol of biological processes for the treatment of wastewater. Thepresent invention seeks to fulfil this need.

SUMMARY OF THE INVENTION

Our invention meets this need in a variety of ways. It includes bothmethods and apparatus. Among these are methods of controlling abiological wastewater treatment process and control system apparatus forcontrolling a biological wastewater treatment process. These comprise anumber of different combinations of devices, steps and conditions, eachof which represents a particular aspect of what we have invented.

A first method aspect comprises, in at least one treatment tankcontaining wastewater, conducting a biological process supported, atleast in part, by introducing oxygen-containing gas into the wastewaterin the form of bubbles provided in the wastewater by a gas supplysystem, and causing at least a portion of the oxygen in said bubbles todissolve in the wastewater. At least a portion of the dissolved oxygento be consumed by the biological process, wherein the oxygen sodissolved may represent an excess or a deficiency relative to the oxygenconsumed by the biological process, and wherein at least one gascollection member is positioned in the treatment tank to receive offgasrepresenting gas from said bubbles that has not been dissolved into thewastewater. Operation of the biological process is controlled with acontrol system that, as the process operates, exercises continuingcontrol over the process at least partially in response to measurementsthat are taken by the control system from the offgas collected in thegas collection member and that are correlative with the amount of one ormore gases in the offgas. The invention utilizes data obtained throughsaid measurements to provide, in the control system, for the varyingamounts of consumption of oxygen that occur in the biological process,control values, or components of control values, that change in responseto, while remaining correlative with, such varying amounts of oxygenconsumption, and generating control signals based on the changingcontrol values or components.

A second method aspect comprises, in at least one treatment tankcontaining wastewater, conducting a biological process comprisingsuspended growth aeration. In this process, biological breakdown ofsuspended and/or dissolved waste material present in the wastewater issupported, at least in part, by introducing oxygen-containing gas intothe wastewater in the form of bubbles provided in the wastewater by agas supply system. These bubbles rise through at least a portion of thedepth of the wastewater in the direction of its upper surface, and causeat least a portion of the oxygen in said bubbles to dissolve in thewastewater and at least a portion of the dissolved oxygen to be consumedby the biological process. The oxygen so dissolved may comprise anexcess or represent a deficiency relative to the oxygen consumed by thebiological process. At least one gas collection member is positioned toreceive offgas representing gas from said bubbles that has not beendissolved into the wastewater. The method controls the operation of theprocess with a control system that, as the process operates, exercisescontinuing control over the introduction of wastewater into the processand/or over the quantity of gas discharged into the tank through saidgas supply system, at least partially in response to measurements of theoffgas, taken by the control system, that are correlative with theamount of one or more gases in the offgas. Data obtained through saidmeasurements is utilized to provide, in the control system, controlvalues which are at least in part correlative with changing needs forthe supply of dissolved oxygen to the wastewater as determined by thecontrol system at least partly on the basis of such data.

A third aspect, which is control system apparatus, comprises at leastone gas collection member that is positioned in at least one wastewaterprocessing tank in which the biological process is conducted, to collectfrom the wastewater in the processing tank, offgas representing at leasta portion of oxygen-containing gas that has been introduced into but notdissolved in the wastewater. There is at least one measuring devicecomprising at least one gas detector that is connected with the gascollection member and that can take measurements and thereby providedata indicative of the amount of at least one gas in the offgascollected by the gas collection member. There is also at least onecontroller connected with the measuring device, which controllerdefines, for the varying amounts of consumption of oxygen that occur inthe biological process, control values, or components of control values,that change in response to, while remaining correlative with, suchvarying amounts of oxygen consumption, which controller generatescontrol signals based on the control values or components.

A fourth aspect involves apparatus of the type that comprises at leastone tank for conducting a biological process comprising suspended growthaeration on wastewater, and a gas supply system for introducingoxygen-containing gas into the wastewater in the form of bubbles andcausing at least a portion of the oxygen in said bubbles to dissolve inthe wastewater and at least a portion of the dissolved oxygen to beconsumed by the biological process. The oxygen so dissolved may comprisean excess or represent a deficiency relative to the oxygen consumed bythe biological process. At least one gas collection member is positionedto receive offgas representing gas from bubbles that have not been notdissolved into the wastewater. This apparatus has a control systemcomprising several parts. there is at least one gas detector that cantake measurements of the amount of at least one gas collected in the gascollection member. There is also at least one DO (dissolved oxygen)detector having a probe that, when in contact with the wastewater in thetank, can take measurements of the DO level of the wastewater. Alsoincluded is at least one controller that contains or has access to codewhich the controller can utilize with said measurements to provide, inthe control system, control values which are at least in partcorrelative with changing needs for the supply of dissolved oxygen tothe wastewater.

The foregoing general methods and apparatus may optionally be practicedin any one or more of the following particular modes, which may involveparticularization of the general methods and apparatus and/or theaddition of steps or other features. The following optional modes,whether employed singly or in any combination, represent not onlypreferred modes of practicing the general methods and apparatus, but,when combined with any of the general methods and/or apparatus, are alsobelieved to be inventions.

A number of these particular modes are applicable to each of the generalmethod and/or apparatus aspects and may be combined with any or allother particular modes. Among these particular modes are those:

-   -   wherein the biological process comprises suspended growth        aeration which includes biological metabolization of suspended        and/or dissolved waste material present in the wastewater is        supported, at least in part, by the oxygen-containing gas        introduced into the wastewater;    -   wherein the biological process is a continuous flow process;    -   wherein the biological process is an activated sludge process;    -   wherein the control system is programmed to tend to maintain a        positive DO level in at least a portion of the tank;    -   wherein said gas collection member is positioned at a surface of        the wastewater;    -   comprising a tank having a wastewater inlet and an outlet, and        the control system includes DO measuring devices at first and        second locations in the tank, the first location being closer to        the inlet than to the second location, or the second location        being closer to the outlet than to the first location;    -   wherein the first location is closer to the gas collection        member than to the second location, or the first location is        adjacent the inlet and the second location is adjacent the        outlet; or the gas collection member and the first location are        each closer to the inlet than to the second location, or the gas        collection member and the second location are each closer to the        outlet than to the first location, or the gas collection member        is positioned between the first and second locations;    -   comprising an elongated tank having upstream and downstream        halves;    -   comprising an elongated tank divided into at least two sections        by a baffle and/or other form of length divider, and at least        one of said sections has upstream and downstream halves;    -   wherein a gas collection member is positioned in an upstream        half of a tank or tank section to receive offgas representing        gas from bubbles that have not been dissolved in the wastewater;    -   wherein the control system includes at least two DO probes        respectively positioned in upstream and downstream halves of a        tank or tank section for gathering data with respect to DO        levels;    -   comprising a tank or tank section having an upstream end, and at        least portions of the gas collection member and of a DO probe        positioned in the upstream half of the tank are respectively        within about the first 10% or 15% or 20% of the length of the        tank, measured from the upstream end;    -   wherein measurements of the offgas taken by the control system        are correlative with the amount of at least one gas representing        at least a portion of the composition of the offgas;    -   wherein the oxygen-containing gas is or comprises air and        measurements of the offgas taken by the control system are        correlative with the amount of oxygen or the amount of carbon        dioxide or the amounts of oxygen and carbon dioxide in the        offgas;    -   wherein a controller contains or has access to code, and        optionally also tables of data, with the aid of which it defines        said control values;    -   wherein the system operates as a feed forward controller where        control outputs are generated, at least in part, based on        requirements control values and performance control values;    -   wherein said control values are requirements control values;    -   wherein said control values comprise requirements control        values;    -   wherein the control values comprise requirements control values        correlative with the oxygen consumed by the biological process,        as determined by the control system;    -   wherein said control values comprise DO control values        correlative with changing amounts of oxygen-containing gas        required to return the DO level in the wastewater to a target        value;    -   wherein said control values comprise performance control values        correlative with variations in the ability of the gas supply        system to transfer oxygen to the wastewater;    -   wherein said control values comprise requirements control values        combined with DO control values;    -   wherein said control values comprise requirements control values        combined with DO rate of change values and DO control values;    -   wherein said control values comprise requirements control values        combined with performance control values;    -   wherein the control system comprises at least one gas quantity        regulating apparatus which, in response to control inputs from        the control system, changes or maintains the quantity of gas        introduced into the wastewater;    -   wherein the control system comprises at least one liquid flow        regulating apparatus which, in response to control inputs from        the control system, changes or maintains the quantity of        wastewater introduced into the tank; and    -   comprising at least first and second tanks, the second of which        is controlled simultaneously with the first tank, or which is        controlled independently from the first tank.

A number of the particular modes are applicable to each of the generalmethod aspects and may be combined with any or all of the otherparticular modes. Among these particular modes are those:

-   -   wherein said excess or deficiency manifests itself as an        increase or decrease in the DO (dissolved oxygen) level of the        wastewater;    -   comprising providing OP (operational performance) data in the        control system;    -   comprising providing PS (performance standard) data in the        control system;    -   comprising providing RSP (relative system performance) data in        the control system that is derived at least in part with PS        data;    -   comprising: (A) causing the control system to take, at one or        more locations in the wastewater, continuing measurements that        are correlative with DO levels in the wastewater differing        positively and/or negatively from a target DO value; (B)        generating, in the control system, DO control values of        magnitude sufficient, when applied in conjunction with        requirements control values, to at least partially offset        deviations of DO level in the wastewater from the target DO        value;    -   wherein the control system generates DO control values        correlative with the amount of oxygen required to move the DO        level in the wastewater to the target DO value;    -   wherein: (A) within at least one tank, the wastewater flows        along a flow path that has upstream and downstream portions, (B)        a gas collection member is positioned along the upstream portion        to receive offgas representing gas from bubbles that have not        been fully dissolved in the wastewater, (C) data with respect to        DO level is gathered from at least two DO probes respectively        positioned along the upstream and downstream portions of the        flow path; (D) the control system establishes, on a continuing        basis, control values for the entire tank that are at least in        part correlative with a combination of (1) changing consumption        of oxygen by the biological process, as measured with the aid of        said gas collection member and (2) said DO level data gathered        from the DO probes positioned along the upstream and downstream        portions of the flow path;    -   wherein: (A) the control system establishes, on a continuing        basis, control values that are at least in part correlative with        a combination of (1) changing consumption of oxygen by the        biological process, as measured with the aid of said gas        collection member and (2) deviations, from a first target value,        of the DO level measured by a DO probe positioned along an        upstream portion of the wastewater flow path, and (B) the        control system adjusts said first target value, on a continuing        basis, with the aid of data correlative with deviations, from a        second target value, of the DO level measured by a DO probe        positioned along a downstream portion of the flow path;    -   wherein the wastewater flows in plug flow;    -   wherein the wastewater flows along a flow path having a        dimension in the direction of wastewater flow that is greater        than its average dimension perpendicular to such direction;    -   wherein: (A) data with respect to the rate of change of DO level        is gathered from at least one DO probe positioned in the tank,        and (B) the control system establishes, on a continuing basis,        control values which are applied to a tank as a whole, said        control values being at least in part correlative with a        combination of (1) changing consumption of oxygen by the        biological process, as measured with the aid of the gas        collection member along an upstream portion of a wastewater flow        path through the tank (2) DO level data gathered from at least        two DO probes respectively positioned along upstream and        downstream portions of the flow path and (3) DO rate of change        data;    -   comprising: (A) causing the control system to take, at one or        more locations in the wastewater, continuing measurements that        are correlative with DO levels in the wastewater differing        positively and/or negatively from one or more target DO        values; (B) causing the control system to take, at one or more        locations in the wastewater, continuing measurements that are        correlative with rates of change of DO level in the wastewater;        and (C) generating in the control system, on a continuing basis,        control values that are at least in part correlative with a        combination of the consumption of oxygen in the biological        process, of said DO levels and of said rates of change;    -   comprising: (A) causing the control system to establish, on a        continuing basis, performance values that are correlative with        the ability of the gas supply system to dissolve said oxygen        containing gas in the wastewater, and (B) causing the control        system, on a continuing basis, to combine said performance        values with requirements control values which are at least in        part correlative with changing consumption of oxygen in the        biological process;    -   comprising generating, in the control system on a continuing        basis, RSP control values correlative with relationships        between (A) OP data, generated by the control system,        correlative with the varying ability of the gas supply system to        transfer oxygen to the wastewater under fluctuating process        conditions, comprising one or more of gas supply system        conditions, wastewater conditions, process conditions, and        atmospheric conditions, and (B) PS data, provided in the control        system, correlative with the ability of the gas supply system to        transfer oxygen to water and/or wastewater under predetermined        standards for said conditions;    -   wherein the control values are established at least in part with        OP data which are provided in the control system and which is        based on one or more of the following: gas supply system        conditions, wastewater conditions, process conditions, and        atmospheric conditions, and wherein said condition/conditions,        including characteristics of any of the foregoing, is/are        determined by the control system;    -   wherein the control values are established at least in part with        PS data that includes OTR: Q (oxygen transfer rate: flow) data        correlative with oxygen transfer rates which the gas supply        system could achieve in clean water at varying rates of flow of        gas through the gas supply system;    -   wherein the control values are established at least in part with        apparent alpha values which are correlative with a ratio        between (a) the rate, as determined by the system, at which the        gas supply system can transfer oxygen to the wastewater and (b)        the rate at which the gas supply system can transfer oxygen to        clean water;    -   comprising: (A) providing, in the control system, OTR: Q (oxygen        transfer rate: flow) control values correlative with oxygen        transfer rates which the gas supply system could achieve in        clean water at varying rates of flow of gas through the gas        supply system; (B) providing, in the control system, apparent        alpha values which are correlative with a ratio between (a) the        rate, as determined by the system, at which the gas supply        system can transfer oxygen to the wastewater and (b) the rate at        which the gas supply system could transfer oxygen to clean        water; and (C) deriving RSP values by combining OTR: Q and        apparent alpha values;    -   wherein apparent alpha values are determined at least in part by        the control system and reflect changes in the condition of the        gas supply system and the wastewater that can affect the amount        of oxygen which the gas supply system can transfer to the        wastewater;    -   wherein control values are applied by the system based at least        in part on process control needs comprising at least one form of        process control needs selected from among process oxygen control        needs, DO level control needs, and performance control needs and        wherein the applied control value is within plus or minus 20%,        more preferably 10%, still more preferably 5% and most        preferably 3%, based on the data available in the system at the        time the applied control value is applied, of a reference        control value which would produce a flow rate of gas and/or        wastewater into the biological process that would precisely        satisfy the particular need or needs;    -   wherein control values are applied by the system based at least        in part on process control needs comprising at least one form of        process control needs selected from among process oxygen control        needs, DO level control needs, and performance control needs and        wherein the control values are applied directly or indirectly to        at least one flow regulating device to provide on a continuing        basis control inputs to said device to cause said device to        change or maintain the quantity of gas introduced into the        wastewater and/or to change or maintain the quantity of        wastewater introduced into the tank;    -   wherein control is effected, at least in part, using data on        rates of change of DO level in the tank over one or more        predetermined time periods;    -   wherein the control system derives control inputs based at least        in part (1) on differences between (a) the actual wastewater        temperature and (b) a selected reference temperature, and/or (2)        on differences between (a) the actual barometric pressure acting        on the wastewater surface and (b) a selected reference        barometric pressure;    -   wherein the control system exercises control at least partially        in response to measurements correlative with OUR (oxygen uptake        rate), or OTR (oxygen transfer rate), or OTE (oxygen transfer        efficiency), or any combination thereof; and    -   wherein the control system derives control inputs by adjusting        the control values at least in part with respect to the control        response characteristics of a flow regulating device;

A number of the particular modes are applicable to each of the generalapparatus aspects and may be combined with any or all other particularmodes. Among these particular modes are those:

-   -   wherein the control system comprises at least one of the        following: a device for measuring wastewater temperature; a        device for measuring gas flow from the gas collection member; a        device for measuring the dissolved oxygen content of the        wastewater; and a device for measuring oxygen content in the        offgas;    -   wherein the control system comprises a device for measuring        wastewater temperature, a device for measuring gas flow from the        gas collection member, a device for measuring the dissolved        oxygen content of the wastewater, and a device for measuring        oxygen content in offgas;    -   comprising code that defines, on a continuing basis, RSP        (relative system performance) control values correlative with        relationships between (A) OP (operational performance) data        correlative with the varying ability of the gas supply system to        transfer oxygen to the wastewater under fluctuating process        conditions, comprising one or more of gas supply system        conditions, wastewater conditions, process conditions, and        atmospheric conditions, and (B) PS (performance standard) data        correlative with the ability of the gas supply system to        transfer oxygen to water and/or wastewater;    -   comprising code that defines OP data;    -   that includes or has access to PS data;    -   comprising code that defines RSP data at least in part with PS        data that is stored in the control system;    -   wherein PS data is stored in the system and includes OTR: Q        (oxygen transfer rate: flow) data correlative with oxygen        transfer rates which the gas supply system could achieve in        clean water at varying rates of flow of gas through the gas        supply system;    -   wherein at least one control element is connected with the        controller and is responsive to the control signals generated in        the controller to effect control over at least a portion of the        biological process by adjusting at least one parameter of the        process;    -   which further includes one or more liquid flow control units        that can control introduction of wastewater into the tank;    -   which further includes one or more gas flow control units that        can control the introduction of gas discharged into the tank        through said gas supply system; and    -   further comprising at least one gas quantity regulating        apparatus capable of changing or maintaining the quantity of gas        introduced into the wastewater, in response to control inputs by        the control system including inputs based at least in part on        requirements control values and DO control values, wherein the        requirements control values and the DO control values are based        at least in part on relationships with RSP values.

ADVANTAGES

Some embodiments of the present invention measure oxygen consumption andthe performance parameters of the aeration system. This provides anopportunity for “predictive” (or feed forward) control where therequired controlled variable (e.g., air flow rate) can be predictedbased on oxygen consumption and equipment performance. It is believedthat, in practice, prior art control systems have almost exclusivelybeen “reactive” (feedback). These prior systems react to errors inprocess performance, and errors are thus an inherent result of certainprior control systems' performance. Because of the errors generallyinhering in feedback systems the biological activity of microorganismsin processes operated under this mode of control can be compromised byfluctuations in the dissolved oxygen level. With preferred embodimentsof our invention, the variables critical to biological activity can bemade more stable, resulting in reduced effluent variations.

To minimize the deleterious impact of errors in prior systems, there isa tendency for operators to set the target dissolved oxygen level at avalue higher than the minimum level which would be acceptable in awell-controlled operation. This provides a “cushion” to preventexcursions in loading from causing excessive decrease in the dissolvedoxygen level. Because the operation of certain preferred embodiments ofour system can be more stable and errors can be minimized or eliminated,the target level of dissolved oxygen can be set lower. This can producehigher efficiency and result in significant savings in energy and otherassociated costs.

Pumping and the time required for reactions to occur in systems thatwithdraw liquid samples, such as most respirometric techniques, resultin a time delay between the beginning of the measurement process andobtaining the results. Because of the construction of preferredembodiments of our invention, it is possible to capitalize on the speedof fast measurement devices leading to near or true “real time”determination of the oxygen requirements and performance of the process.

Some preferred embodiments of our invention monitor the impact ofchanges in oxygen consumption in real time. These embodiments afford anopportunity to detect slug loading or inhibitory contaminants fromindustrial contributors or other sources. The rapid response of theseembodiments will minimize the impact of these changes on the effluentquality and alert the operator so proper corrective measures can beimplemented.

Some existing systems measure the oxygen demand of the wastewater.Contrary to what is common in prior practice, certain embodiments of theinvention can measure the performance of the aeration equipment (i.e.diffusers) on a continuing basis and even in real time. These measuredparameters may for example include oxygen transfer efficiency and alpha(ratio of actual process to clean water performance). The presentcontrol system can apply apparent alpha values, determined by thecontrol system, that reflect changes in the condition of the gas supplysystem and the wastewater that can affect the amount of oxygen which thegas supply system can transfer to the wastewater. This informationprovides insights into actual aeration system performance and affords anopportunity to monitor degradation of the system over time due tofouling and/or other forms of degradation of aerator performance.Cleaning or replacement of diffusers can be optimized based on actualperformance, minimizing the costs of premature or unduly delayedcleaning or replacement, thus permitting cleaning before performance andenergy efficiency is significantly degraded.

In the tuning of certain prior systems, system response to errors andload changes is monitored and the parameters affecting response aremodified by empirical results derived from observation and experience.For example, this is true of certain “PID”(Proportional-Integral-Derivative) control algorithms, but it is alsogenerally true of feedback control algorithms. Changes in aerationsystem condition, incoming waste and ambient conditions requiredmodification of the tuning parameters. Because in certain of itspreferred embodiments our system's response is based on the physicalconfiguration of the process equipment and a combination of known andmeasured aeration system effectiveness, the tuning is insensitive tochanges in aeration system condition, incoming waste and atmosphericconditions.

Once data on the physical configuration and aeration system performanceis stored, these embodiments can predict the response to theabove-mentioned changes by mathematical calculations based on knownperformance parameters.

Certain prior control systems have used “lumped parameter” tuning, wherethe effects of process loading, biological performance and aerationsystem performance are not differentiated in determining the response ofthe system to perturbations. A change in process parameters required achange in the tuning of the control system. With certain embodiments ofour control system process parameters related to process loading,biological performance and aeration system performance are individuallymonitored, making such systems both more responsive and more stable.

A number of existing methods used to measure oxygen requirements of atreatment system, such as most respirometric techniques (also referredto as respirometry), involve movement of samples of the contents of theaeration tanks to a reaction cell. In many systems additional chemicalsmust be used to determine the oxygen requirements of the wastewater. Thepumping and fluid handling systems are prone to plugging and requiresignificant maintenance. The additional chemicals, if required, are anadditional cost of operation. Because preferred embodiments of ourinvention use gas leaving the surface rather than withdrawn liquidsamples, it is not prone to such plugging and maintenance is minimized.Reliability is also enhanced.

All embodiments of the invention, whether specifically disclosed hereinor not, will not necessarily have all of the above advantages, nor thesame combinations of advantages. Moreover, users of the invention,manufacturers of components or complete systems involving the inventionand other persons skilled in the art may identify, with the aid of thepresent disclosure and/or through experience with the invention,embodiments that inherently include advantages not discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 are each schematic diagrams of biological wastewater treatmentprocesses and components of control systems according to the invention.

FIG. 5 is a flow sheet illustrating the data entry functions and controllogic functions of software useful in practicing the invention.

VARIOUS AND PREFERRED EMBODIMENTS Introduction

In general, our control method and apparatus are useful with a widevariety of biological wastewater treatment processes. Typically, theseare processes in which aeration with oxygen-containing gas supports themetabolizing of waste by bacteria in the wastewater, e.g., activatedsludge processes, in one or more tanks. Other gases or vapors may beused in or in connection with these processes for any suitable purpose,e.g., cleaning gas.

Our control system employs any form of measurement apparatus to receivedata on one or more process parameters, which may include any parametersof or affecting the process. Such parameters include varying amounts ofone or more gases in offgas recovered from the wastewater in the tank,and may include, for example, other gas and liquid flows, watertemperature, atmospheric pressure and other variables. Measurements ofthese parameters may be made by any suitable kind(s) of measurementdevices. They are connected with, and are used to furnish needed data onprocess parameters to, a controller.

The data outputs of the measurement devices to the controller, whetherin electrical or other form, need not correspond directly, e.g., benumerically proportional with, measured parameters expressed incustomary units. However, for at least some measurements of interest,measurement devices are available that give outputs correspondingdirectly with measured parameters, and these devices are preferred.

The controller employs the data outputs to establish varying controlvalues correlative with, among a variety of possibilities, one or morevarying process needs for oxygen. These include requirements controlvalues, and may also include DO control values and/or performancecontrol values. How this is accomplished can depend to some extent onthe nature of the measurement device outputs and/or the capabilities ofthe controller.

Whether the data outputs do or do not directly correspond with themeasured parameters, the controller may for example contain or haveaccess to, and derive any of the control values from, a table whichcontains and matches data output values with appropriate, precalculatedcontrol values. On the other hand, the controller may calculate any ofthe control values from algorithms, as data is received, where directlycorresponding data outputs are available to it for the parameters neededin the calculation. Calculation of control values as data is received isalso possible if directly corresponding data outputs are not availableto the controller, for example when it contains or has access to meansfor converting those data outputs to a form useful in such calculations.Detailed information on calculation of control values is provided below.

Varying control values, present in the controller, are used, with orwithout adjustment, to provide control signals in the controller. Anysuitable kind of automated control element(s), such as control valves,weirs, motor controls and other devices, is/are connected with thecontroller, which transmits the control signals to them. The signals maybe the control values themselves or may differ from them. For example,the control values may have been adjusted in generating control signals,e.g., to conform with signal requirements of the control elements orwith such factors as the operational characteristics of those elements,of the gas supply system or of the process.

Preferably, the control values directly correspond numerically with theprocess need or needs to which the control values relate, and thesignals have adjusted magnitudes which provide some selected incrementof the control action required by the control values and the relatedneed. Then, as the system takes continuing measurements of processparameters affected by the incremental control action, the controlvalues may remain the same or be changed by the controller as a resultof observation of the effects of the control action applied and/or ofother factors. Further control signals of the same or differentmagnitude as previous ones may then be issued to the control element(s)to continue the control action in increments for satisfying the thencurrent control values.

Separate control signals may be issued by a controller or controllers,separately representing different kinds of control values. For example,varying, separate signals may be transmitted to separate, plural gassupply control elements, which signals are respectively based on varyingrequirements control values and varying DO control values. Then, basedon the separate control signals the separate control elements can supplyseparate, regulated flows of gas from separate gas supply lines. Theseflows can enter a treatment tank as separate flows or, after having beencombined with each other upstream of the tank but downstream of thecontrol elements, as a single flow. The aggregate amount of theseseparate flows, whether entering the tank in the form of single orplural flows, can be in an amount sufficient to meet the varying needsfor oxygen to metabolize, and, optionally, to otherwise treat, waste inthe wastewater and to maintain a substantially stable DO level.

However, the controller preferably generates varying control values ofwhich two or more different kinds of control values are component parts,such as a combination of varying requirements control values and varyingDO control values. Then, the controller may, if desired, generatevarying control signals correlative with a varying combination or totalof the different control value components. These signals may if desiredbe transmitted to a single control element. In turn, such a controlelement may if desired cause a single gas line, or a combined set of gaslines, to provide gas to the wastewater in the amounts needed.

When, as preferred, the varying control values used to generate controlsignals include as component parts requirements control values, DOcontrol values and performance control values, gas may then be suppliedin the varying amounts required to meet the need for oxygen tometabolize or otherwise treat waste, suitably adjusted to maintainstable DO levels and account for performance changes. Performancechanges may for example involve one or more of the following: gas supplysystem conditions, e.g., the results of diffuser fouling, diffusercleaning or changes in gas supply rates and the resulting changes indiffuser flux rate where area-release fine bubble diffusers areinvolved; changes in wastewater conditions; variations in processconditions, and changing atmospheric conditions. Suchcondition/conditions, including characteristics of any of the foregoing,is/are, or may be, as determined by the control system.

Whatever the nature and mode of use of the control values and controlsignals, the control system causes the control elements to act inresponse to those signals for effecting control over the biologicalprocess. The control system may effect control over the biologicalprocess in any way that is effective in matching the availability ofoxygen-containing gas to the changing consumption of or need for oxygenin all or a portion of the process, and possibly for meeting otherneeds.

Examples of ways of effecting control over the process include one ormore of the following: turning up or turning down the flow of gas and/orwastewater to the process, changing the distribution of gas introducedinto the system, changing the quantity or distribution of wastewater inthe tank, e.g. as in step feeding, changing the operating intensity ofmechanical or brush aerators, turning at least a portion of themechanical or brush aerators and/or diffusers that are available in thesystem on or off, feeding zero or varying amounts of supplemental oxygento the process, and altering the oxygen transfer efficiency of theoperation, such as by changing the distance traversed by gas bubbles asthey pass through the wastewater, e.g., by turning agitators on, up,down or off and/or altering the wastewater depth in a given tank.Control elements will be selected that are suitable for the chosenway(s) of effecting control over the process.

The following discussions and FIGS. 1-5 present several specific,illustrative embodiments of wastewater treatment apparatus, controlsystem apparatus and software that are useful in the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 schematically illustrate exemplary biological processequipment including tanks and means for introducing oxygen-containinggas into wastewater in the tanks. These figures also illustrate controlsystem arrangements that are compatible with the process equipmentlayouts and that include measuring apparatus to derive data andcontrollers to derive control inputs for the process. FIG. 5schematically illustrates one example of many possible arrangements offunctions within the process and particularly within software thatskilled programmers can design for use in controllers carrying out thepresent invention, such as the embodiments of FIGS. 1-4.

FIG. 1

The embodiment of FIG. 1 includes tank 2 which contains wastewater inwhich a suspended growth aeration process is being conducted. Inlet 17and outlet 18, respectively, are provided for entry of wastewater to beaerated in the tank and discharge of mixed liquor to subsequentprocesses.

In and around the tank are components of a gas supply system. Amongthese are plural devices 3 of any suitable type for introducingoxygen-containing gas bubbles into the wastewater, e.g., fine bubblediffusers, a source of gas 4, which is shown as a pipe but could beanother device and gas flow regulating device 1, which is shown as avalve, but could be another device.

In the process, which may aerate the wastewater continuously orintermittently, bubbles of oxygen-containing gas, generated by the gassupply system, rise through at least a portion of the depth of thewastewater in the direction of its upper surface. Oxygen in the bubblesdissolves in the wastewater. At least a portion of the dissolved oxygenis consumed by the biological process. The oxygen so dissolved maycomprise an excess or represent a deficiency relative to the oxygenconsumed by the biological process. Such excess or deficiency maymanifest itself as an increase or decrease in the DO (dissolved oxygen)level of the wastewater.

A control system according to the invention controls the aerationprocess. In this embodiment, it includes a device 5, for measuringwastewater temperature, a gas collection member, e.g., a hood 10 forcollecting gas escaping from the tank, a device 11 for measuring gasflow from the hood, a device 12 for measuring the dissolved oxygencontent of the wastewater, a device 13 for measuring oxygen content inoffgas, a controller 14 for automatically executing control logic,connections 15 for transmitting measured values to the controller andcontrol signals from the controller and an outlet 16 for dischargingsample air to the atmosphere.

In the embodiment illustrated in FIG. 1 hood 10 represents a locationfrom which to obtain data useful to determine the estimated oxygentransferred by the gas supply system. Probe 12 represents a locationfrom which to obtain data to determine the estimated DO level in thetank.

From this data, controller 14 establishes corresponding requirementscontrol values which are correlative with the oxygen transferred by thegas supply system and the rate of increase/decrease of DO, as determinedby the control system. Controller 14 also establishes DO control valuescorrelative with the oxygen adjustment required, as determined by thecontrol system, to return DO levels to a target value. Preferably, theestablishment of requirements control values is at least partially inresponse to measurements correlative with the OUR (oxygen uptake rate)of the wastewater, or the OTR (oxygen transfer rate) of the gas supplysystem, or the OTE (oxygen transfer efficiency) of the gas supplysystem, and preferably some combination of these.

Preferably, the requirements control values correlative with the oxygenconsumed by the biological process, as determined by the control system,are combined with DO control values. The controller combines these twokinds of control values, whether additive or offsetting to some extent,and from this total establishes control values and corresponding controlsignals that, with or without adjustment, e.g., to account for theresponse characteristics of the valve actuator, are transmitted bycontroller 14 to gas flow regulating valve 1. Where the process is acontinuous flow process, the combination of control values generated bythe control system may be correlative with a combination of rates ofoxygen consumption and rates of change of DO level in the tank over oneor more predetermined time periods. Typically, the control system isprogrammed to tend to maintain a stable, positive dissolved oxygen levelin at least a portion of the tank, while meeting the varying oxygenneeds of the biological process.

The selected locations for the hood and probe may be arbitrary if thetank contents are substantially completely mixed and homogenous, or, ifnot, may be locations of specific interest to the operator.

FIG. 2

Here again, a control system according to the invention controls theaeration process in a plural tank aeration operation. In common with theFIG. 1 embodiment, this embodiment has a first tank 23 which containswastewater in which a suspended growth aeration process is conducted.Inlet 78 and outlet 79, respectively, are present for entry ofwastewater into the tank and discharge of mixed liquor.

In and around this tank are components of a gas supply system. Amongthese are a source of gas 25, which is shown as a pipe but could beanother device, and plural devices 24 of any suitable type forintroducing oxygen-containing gas bubbles into the wastewater.

The FIG. 2 embodiment includes a second aeration tank 45 which containswastewater undergoing suspended growth aeration. Inlet 78 and outlet 79,respectively, are provided for entry and discharge of wastewater andmixed liquor.

Blower or compressor 75 supplies air or gas to tank 45 and optionally toone or more additional tanks. Plural devices 24 of any suitable type arepresent in tank 45 and are connected to the blower for introducingoxygen-containing gas bubbles into the wastewater.

There are three sampling lines 41, 42 and 43. They respectively includecertain auxiliary devices, discussed below. Lines 41 and 42 draw gasesfrom the hoods 32, which are part of measurement apparatus to bediscussed further below, to determine requirements control values forthe tanks 23 and 45. Hood 32 of tank 45 has a flotation device 46 tomaintain the hood at the wastewater surface during water levelvariations. Line 43 and its auxiliary devices draw ambient air from theatmosphere through intake 39 for calibrating and verifying the accuracyof the measurement apparatus.

Auxiliary devices present in all three sampling lines includecompressors 49 to provide positive flow of offgas from the hoods 32through the sampling lines to the measurement apparatus for analysis,pressure relief valves 50 to prevent build-up of excessive pressure inthe lines, drying devices 55 to remove entrained water and water vaporfrom the gas in the lines prior to its entry into the measurementapparatus and valves 56, which may be other kinds of devices. Thesevalves control the direction of gas and/or gas flow in proper sequencefrom various tanks to the measurement apparatus and/or to theatmosphere.

Optionally, several additional system elements may be provided. Forexample, line 42 may have a discharge conduit 47 to release excess gasfrom the hood 32 of tank 45 into or adjacent to the wastewater andheating system 48 to prevent condensation of water vapor. Lines 57 mayprovide entrances for gas from other hoods or tanks into line 42 and themeasurement apparatus.

Some elements of the measuring apparatus of this embodiment of thecontrol system are arranged along analysis line 44. Device 65 detectsmoisture or condensate in offgas or ambient gas flow. Device 68 measuresgas temperature, while device 67 measures gas pressure. Device 66measures carbon dioxide content in the offgas. Restriction 64 throttlesgas flow to create positive pressure in the measurement system. Device35 measures oxygen content in offgas, while outlet 38 discharges usedsample air to the atmosphere.

Other elements of the measuring apparatus include devices 26 in eachtank, for measuring wastewater temperature, the above-mentioned hoods 32for collecting gas escaping from the tanks, a device 33 in tank 23 formeasuring gas flow from the hood, device 74 for measuring gas flow intotank 45 and device 34 in each tank for measuring the dissolved oxygencontent of the wastewater.

In the embodiment shown in FIG. 2 the arrangement in tank 23 differsfrom the arrangement in tank 45 in the technique employed for measuringthe gas flow to the respective tanks. In tank 23 device 33 is used formeasuring the gas flow escaping from the hood, and this gas flow rate isextrapolated to encompass the entire gas flow to the tank by the ratioof the hood surface area to the area of the entire tank. In tank 45device 74 is used for measuring the gas flow to the entire tankdirectly. Factors bearing on deciding which arrangement to use in agiven tank include the extent of any variation in the process from onelocation to another within the tank, and the nature of existinginstrumentation associated with the tank when converting to the use ofthe present invention. As the figure shows, these two arrangements maybe used in different tanks of the same plant or may be used incombination with each other within the same tank.

With the aid of data from the measuring apparatus a controller 36automatically executes control logic for each tank. Interface device 76is provided to display measured and calculated data and to assist inentering constants and control parameters for operating the system.Connections 37 transmit measured values to the controller and controlsignals from the controller for tanks 23 and 45. Through signals sentvia these connections the controller adjusts the gas flow to tank 23with gas flow regulating device 22, which is shown as a valve, but couldbe another device, and adjusts gas flow to tank 45 by altering the speedof blower 75. Connections 58 transmit measured values from othermeasurement apparatus to the controller and control signals from thecontroller for other hoods or tanks, where such are provided.

As shown by FIG. 2 and the above discussion, each of tanks 23 and 45 hasone point of gas flow entry and control. As in FIG. 1, each hood 32represents a location from which to obtain data useful to determine thevarying amounts of oxygen transferred by the gas supply system for eachtank.

First tank 23 has a DO sensor, device 34, located at the upstream end ofthe tank. Second tank 45 has first and second DO sensors, devices 34 and77, located respectively at the upstream and downstream ends of thattank. The DO sensors, whether or not single or dual sensors are used inthe second tank, provide data on a continuing basis concerning varyingDO levels in their respective tanks. Such data is useful to provide DOrate of increase/decrease data and to determine DO control values thatare correlative with the varying oxygen adjustment required, asdetermined by the control system, to return DO levels to a target value.

From this data, controller 36 establishes, individually for each tank,requirements control values which are based on the oxygen transferred bythe gas supply system and the rate of DO increase/decrease, in therespective tanks, as determined by the control system. From acombination of the requirements control values and the DO control valuesfor each tank, controller 36 establishes separate and varying gas ratesand corresponding control signals that will satisfy varying anddiffering needs for oxygen in the respective tanks. These separatesignals are sent to gas flow regulating device 22 and to blower 75 asrequired to meet such needs. The calculations for each tank may beperformed sequentially or simultaneously in a single controller or maybe performed in a separate controller for each tank.

FIG. 3

In common with FIG. 1, the embodiment of FIG. 3 has a tank 91 whichcontains wastewater in which a suspended growth aeration process isbeing conducted. Inlet 156 and outlet 157, respectively, are providedfor entry of wastewater to be aerated in the tank and discharge of mixedliquor. The tank has at least two distinct zones in which gas flow maybe controlled independently. Here again, a control system according tothe invention controls the aeration process.

In and around the first control zone of tank 91 are components of a gassupply system. Among these are plural devices 92 of any suitable typefor introducing oxygen-containing gas bubbles into the wastewater, asource of gas 93, which is shown as a pipe but could be another deviceand gas flow regulating device 90, which is shown as a valve, but couldbe another device. The gas supply system of the second control zone ofthe tank is also served by the gas source 93 and is provided with a gasflow regulating device 95 and plural devices 96 introducingoxygen-containing gas bubbles into the wastewater.

The first control zone of the tank, in common with FIG. 1, includes adevice 94, for measuring wastewater temperature, a gas collectionmember, e.g., a hood 110, for collecting gas escaping from the tank, anda sampling line 122. Arranged along sampling line 122 are measuringdevices and several auxiliary items, discussed below, and a device 111,for measuring gas flow from the hood.

Associated with the second control zone are a device 97, for measuringwastewater temperature, a hood 118, for collecting gas escaping from thetank, and a sampling line 123. Along line 123 are a device 119, formeasuring gas flow from the hood, and various auxiliary devices.

A third sampling line 124, which also includes auxiliary devices, isalso installed. It receives ambient air from intake 117 for calibratingand verifying the accuracy of the system.

The auxiliary devices in lines 122, 123 and 124 include compressor orcompressors 127 to provide a positive flow of offgas from hood 110, hood118 and intake 117 through these lines. Pressure relief valves 128prevent build-up of excessive pressure in the lines. Drying systems 133remove entrained water and water vapor from offgas. Valves 134 or otherdevices control flow of air and/or other gas from the hoods or intakeinto a measurement system that includes sample analysis line 125.

On line 125 are found a restriction 142 which throttles gas flow tocreate positive pressure in the measurement system and a device 143which detects moisture or condensate in offgas or ambient gas flow.Device 144 measures carbon dioxide content in the offgas. Device 145measures gas pressure, device 146 measures gas temperature. Device 113measures oxygen content in the offgas, and an outlet 116 dischargessample air to the atmosphere.

Other elements of the measurement system include devices 152 and 153which measure gas flow to the first and second zones of the tank, aswell as devices 112 and 120 for measuring the dissolved oxygen contentof the wastewater in the first and second zones.

The control system includes a controller 114, for measurement andprocess control. It automatically executes control logic for both zonesof the tank. Connections 115 transmit measured values from themeasurement system to the controller and control signals from thecontroller to valves 90 and 95. Interface device 154 can assist in entryof constants and control parameters into the system and displaysmeasured and calculated data.

In the embodiment illustrated in FIG. 3 each portion of the tankconstitutes a separate zone of operation, with the ability to measureand control gas flow in each of the zones independently of the otherzone. It is usual, but not mandatory, that the tank will be configuredas a plug flow tank so that the flow of wastewater under treatment willbe from the first zone into the second zone, with or without otherintervening zones. In this embodiment requirements control values foreach zone are calculated independently of all considerations of previousor subsequent zones. Similarly, DO control values for each zone arecalculated independently of all considerations of previous or subsequentzones. This is true whether or not the calculations for each zone areperformed sequentially or simultaneously in a single controller or thecalculations are performed in a separate controller for each zone.

From a combination of the requirements control values and DO controlvalues for each zone, controller 114 establishes separate and varyinggas rates and corresponding control signals that will satisfy thevarying needs for oxygen in the respective zones. These signals are sentto gas flow regulating devices 90 and 95 as required to meet such needs.

Additional embodiments implicit in the arrangement identified in FIG. 3and employing the principles illustrated therein would include more thantwo separate control zones in a single plug flow tank. or two or moreseparate control zones in parallel plug flow tanks. The principlesillustrated by FIG. 3 are further independent of whether or not bafflesor tank walls separate control zones.

FIG. 4

The embodiment of FIG. 4 will be preferred for many wastewater treatmentplants where economic considerations, pre-existing tank configurations,and/or process considerations dictate a system simpler than that shownin FIG. 3 but more complex than that shown in FIGS. 1 and 2. In commonwith FIG. 1, the embodiment of FIG. 4 has a single tank 170 conducting asuspended growth aeration process, inlet 228 and outlet 229,respectively, for wastewater entry and mixed liquor discharge, and onelocation for sampling gas escaping from the tank, but two locations fordetermining DO levels.

In and around tank 170 are components of a gas supply system. Amongthese are plural devices 174 of any suitable type for introducingoxygen-containing gas bubbles into the wastewater, a source of gas 175,which is shown as a pipe but could be another device, and a gas flowregulating device 180.

In common with FIG. 1, this control system includes sampling lines 177and 178 and analysis line 179. These lines include or are connected withvarious items of auxiliary devices or measuring apparatus, discussedbelow.

Lines 177 and 178 include such auxiliary devices as compressors 199 toprovide a positive flow of gas, pressure relief valves 200 to preventbuild-up of excessive pressure, drying systems 205 to remove entrainedwater and water vapor, and valve 206 or some other device to control thedirection of gas and/or air flow from various locations, to theatmosphere and/or to the measurement apparatus in proper sequence. Line178 also includes an ambient air intake 189, for calibrating andverifying the accuracy of the measurement apparatus.

Among the elements of the measuring apparatus in and around the tank area device 176, for measuring wastewater temperature, and a gas collectionmember, e.g., a hood 182, connected to sampling line 177, for collectinggas escaping from the tank. Also, line 177 includes a device 183, formeasuring gas flow from the hood.

Arranged along analysis line 179 are device 215 which detects moistureor condensate in offgas or ambient gas flow, device 218 which measuresgas temperature, device 217 which measures gas pressure, device 216which measures carbon dioxide content in the off-gas, device 185 whichmeasures oxygen content in offgas, restriction 214 which throttles gasflow to create positive pressure in the measurement apparatus, and anoutlet port 188 which discharges sample gas to the atmosphere.

Other elements of the measurement apparatus include device 222 thatmeasures flow from gas source 175 to tank 170 and two DO sensors 184 and192. These measure the DO level of the wastewater in upstream anddownstream portions of the tank, respectively.

Additional elements of the embodiment of the control system include acontroller 186, for automatically executing control logic, andconnections 187, for transmitting measured values to the controller andcontrol signals from the controller to valve 180. Interface Device 226is provided to display measured and calculated data and to facilitateentry of constants and control parameters for operating the system.

It is often important to maintain the DO level in a particular portionof a tank at a fixed or substantially uniform level to ensure that thewastewater oxygen demands have been satisfied. Generally, but notnecessarily, DO level is monitored for this purpose downstream of thepoint of entry of the wastewater, near where the wastewater flows out ofthe tank. The DO level in this portion of the tank can be critical fordetermination of DO control values. Thus, FIG. 4 shows the downstream DOsensor, device 192, at or near the downstream end of the tank.

However, it is often true, especially in a plug flow operation, that thedownstream end of the tank is not the optimum place for gathering dataon the oxygen requirements of the process to determine requirementscontrol values. Thus, in this embodiment, other components of thecontrol system of the invention are located elsewhere, typically but notnecessarily near the location where wastewater flows into the tank.

Accordingly, in the present embodiment, hood 182 and related controlcomponents are positioned near wastewater inlet 228, upstream of thelocation in the tank at which it is desired to maintain a specific fixedor substantially uniform oxygen level. Sensors used to determinerequirements control values, e.g., device 185 to measure the oxygencontent in the offgas and device 184 to measure DO at this location, arelocated in a way that they can determine the oxygen requirements at theupstream location.

Controller 186 uses data from devices 184 and 185 at the upstreamlocation to calculate the varying requirements control values that willsatisfy the need for varying amounts of oxygen to support metabolizationof waste. The controller also calculates, based on data from DO sensor192, varying DO control values necessary to maintain specified target DOlevels in the downstream location. These requirements and DO controlvalues are combined in the controller to establish varying total gasflow rates for the tank as a whole that are estimated to be necessaryfor satisfying steady state and dynamic needs for oxygen to supportmetabolization and DO control needs. Such gas flow rates are used by thecontroller to establish varying control signals sent to control valve180 when and as needed to satisfy such needs.

An alternate mode of operation for the apparatus shown in FIG. 4 is toutilize the upstream location for determining both DO control values andrequirements control values for the entire tank. In this configuration,referred to as “cascade control”, DO control values are calculated fromthe dissolved oxygen level measured at the upstream location by device184. The DO target level used for determining DO control values for theupstream location is calculated from DO levels measured at thedownstream location by device 192. A mathematical function can be usedto establish a relationship between downstream DO levels and upstream DOtarget levels. For example, one may use a ratio of one level to theother, or a ratio of (a) the difference between a downstream target DOlevel and downstream measured DO levels to (b) the upstream targetlevel. Alternatively, the upstream DO target level may be increased ordecreased as the measured downstream DO level falls below or above thedownstream target DO level. From such functions, a controller maycalculate DO control values to combine with requirements control valuesto control gas flow into the tank.

Further embodiments include, among others, a number of tanks, each witha single point of gas flow control, utilizing a single controller and asingle set of devices for measuring the characteristics of the gasleaving the process. Each of these tanks may or may not have a separateset of devices for determining downstream DO control values and/orupstream requirements control values.

FIG. 5

FIG. 5 is a schematic software and process flow sheet. Within the dataentry functions area, located at the upper left corner of the flowsheet, are three parallelograms identifying data to be entered into andstored in the memory of controllers when the system is set up. This datamay be updated from time to time if necessary. Within the control logicfunctions area is a series of rectangular boxes and parallelograms thatidentify operations that are performed by the controllers.

A first parallelogram located in the upper left corner of the controllogic functions area identifies inputs of data into the controller fromsensors in the control system, e.g., wastewater temperature measuringdevice 5, gas flow measuring device 11 and oxygen content measuringdevice 13 of the FIG. 1 embodiment. Based on continuous or periodicmeasurements taken by whatever sensors may be included and active inthat embodiment or other embodiments of the control system, thecontroller generates, on a continuing, e.g., repetitive, basis, varyingcontrol values, e.g., requirements control values, DO control values andsuch other control values as may be desired.

Another parallelogram in the lower right corner of the control logicfunctions area represents repetitive outputs of the controller to gasquantity regulating apparatus, such as one or more of the valves 1illustrated in FIG. 1. Such controller outputs represent control inputsfrom the control system to the aeration process, causing a valve orother device, e.g., valve 1 of FIG. 1, to act in response to such inputsand change or maintain the quantity of gas introduced into thewastewater.

In the present control system embodiments, the controller has a built inor operator selectable waiting time. This is an increment of time thatmay be selected to elapse between repeated controller outputs and basedfor example on anticipated or observed system response time(s), and/orthe degree of fineness of control desired and/or other considerations.See the box above the third and fourth columns in the control logicfunctions area. These increments may be of any suitable duration.

Embodiments such as that illustrated by FIG. 5 include provision forcorrecting, over a selected response time, such errors as may exist inthe DO level of the wastewater, thus tending to move the DO level backtoward a target value. In such embodiments, waiting time is preferablycoordinated with system response time so that the entire response timesubstantially coincides with or occurs within the waiting time. However,commonly used continuous output controls (such asProportional-Integral-Derivative) may be employed with or withoutwaiting time without departing from the fundamental principles of theinvention.

Persons skilled in the art will understand that the order of a number ofthe functions in the flow sheet may be rearranged, and that the controlsystem can nevertheless operate successfully. Furthermore, personsskilled in the art will readily perceive that it is possible to designembodiments that involve modification or elimination of some steps inthe flow sheet without departing from the fundamental principles of theinvention.

In the foregoing preferred embodiments, the varying control values, orcomponents thereof, remain correlative with the varying amounts ofoxygen consumption by the biological process. That is, there is anestablished quantitative relation, present in the control system,between requirements control values and such consumption. Thisquantitative relation is applied repeatedly by the control system in thedetermination of changing requirements control values during a givenrise and/or fall of such consumption detected by the system. Suchrepetition preferably occurs during a plurality of consecutivedeterminations of requirements control values during the given riseand/or fall. Still more preferably, repetition in plural consecutivedeterminations occurs during a plurality of consecutive rises and falls.However, the above-mentioned quantitative relation may be changedintentionally from time to time, such as by an operator and/or by thecontrol system itself, e.g., by an adaptive controller, to refine thematch which the system makes between requirements control values andoxygen consumption.

Additional Embodiments

There are a number of additional embodiments which may optionally bepracticed in conjunction with the embodiments described above, or withother embodiments of the invention. These include, by way of example andnot limitation:

1) Measuring O₂ concentration as % of volume or % of mass directly in anoffgas stream.

2) Adding CO₂ concentration as % of volume or % of mass measurement tothe sample gas stream to increase accuracy of determination of offgasoxygen concentration.

3) Establishment of DO control values related to gas flow required toaccount for DO error based on the equation:

${\frac{\Delta\;{{DO} \cdot V}}{t_{c}} \cdot \frac{1}{\alpha\;{F \cdot \theta^{\prime} \cdot C^{\prime}}}} = {{Gas}\mspace{14mu}{Flow}\mspace{14mu}{Required}}$Where:

-   ΔDO=DO_(target)−DO_(actual)-   V=Control volume, may refer to the complete tank or part of it-   t_(c)=Time constant to establish the time set to correct actual DO    to Target DO-   αF=Apparent alpha value, combined effect of wastewater    characteristics (α), and gas supply system condition (F), on gas    supply system ability to transfer oxygen to wastewater-   θ′=Correction factor for effect of wastewater temperature on gas    supply system ability to transfer oxygen to wastewater    θ′=θ^((T-20)), where T is wastewater temperature-   θ=Arrhenius coefficient for wastewater temperature correction factor    to account for wastewater temperature effect on oxygen transfer-   T=Wastewater Temperature-   C′=Correction factor to account for effect of DO levels or Target DO    levels on the ability of gas supply system to transfer oxygen to    wastewater

$C^{\prime} = \frac{C_{\infty\; f}^{\star} - {DO}}{C_{\infty\; 20}^{\star}}$4) Periodically drawing a sample of ambient air and using the results tocorrect for drift and calibration error in the offgas O₂ concentrationand CO₂ concentration measurement devices.5) Using a positive displacement compressor on the sample gas line tomaintain constant sample time latency and insure constant sample flowrate.6) Using pressure and temperature measurement on the sample gas line orhood exhaust line to convert volumetric flow rate to mass flow rate.7) Using a direct mass flow measurement device to measure mass directly.8) Adding multiple reactors and hoods to be sampled and wherein controlaction is determined in a specific sequence by a single controller.9) Measuring barometric pressure to increase the accuracy of thecalculations.10) Calculating and displaying values derived from measured data thatare of use in monitoring reactor performance, including for example:

αF, K_(La), airflow/diffuser, SOTR and OUR.

11) Plotting and archiving performance data over time.

12) Providing alarming for excursions in process parameters to indicateequipment failure, process problems, and maintenance requirements.

13) With ambient calibration systems or multiple tank systems, addingsolenoid valves to vent gas and allow continuous sample compressoroperation.

14) Integrating the control of a single reactor with the control of acomplete system and with control of blowers to coordinate all controlactions and minimize perturbations.

15) Using a single modulated blower for each reactor instead of multiplereactors drawing gas from a common distribution system.

16) Applying empirically derived constants to the oxygen-containing gasflows to accelerate or decelerate system responses or offsetsite-specific conditions; for example such constants may be applied toflows determined from DO control values if process considerationsrequire the response time to differ from theoretical values, or tocorrect short term sags in DO levels.

Preferred Embodiment of Control System Calculations

Further discussion which follows includes a preferred embodiment ofbases for calculations that are useful in generating control values andare thus useful in constructing appropriate software or code for thecontroller. This discussion describes a preferred embodiment of how tocontrol a diffused air aeration gas supply system in such a way so that:

-   -   1. Requirements control values are developed in order to satisfy        the oxygen requirements of the biological treatment process, and    -   2. DO control values are developed in order to maintain a        pre-established DO concentration at selected locations in the        aeration basin.        A number of process variables and conditions are considered and        manipulated to allow identification of gas supply system        operating settings that will achieve the established objectives.

The basic structure of the preferred control strategy proposed comprisesthe following steps:

-   -   1. Determination of actual oxygen consumption (Oxygen Uptake        Rate or OUR) in the tanks, tank, tank zone or other container of        wastewater under treatment, also referred to as the reactor;    -   2. Determination of oxygen transfer characteristics that will        affect the ability of the gas supply system to supply oxygen to        the process;    -   3. Establishment of oxygen requirements for the process;    -   4. Establishment of required gas supply system operating        conditions to satisfy the requirements established; and    -   5. Adjustment of gas supply system operating conditions to        established conditions.        Thus, the control system includes elements and devices capable        of performing these steps. These steps, and the underlying basis        for the method by which this embodiment performs them, will be        described in greater detail below.

Determination of Actual Oxygen Consumption in the Reactor (OUR)

The Oxygen Uptake Rate (OUR) of mixed liquor is determined using off-gasanalysis and typically is useful in establishing requirements controlvalues within the control system. This methodology allows using at leasta portion of the activated sludge reactor itself as a respirometer, withperformance on a continuing basis of gas phase mass balances of oxygenacross a selected control volume, to determine the amount of oxygenintroduced by the gas supply system, and with performance on acontinuing basis of liquid phase mass balance of oxygen across the samecontrol volume, to identify how much of the oxygen introduced by the gassupply system is being consumed by microorganisms in the wastewater.

The control volume could be the entire liquid-containing volume of thereactor, but is conveniently a selected small portion of the totalvolume selected to provide the most useful or convenient information forcontrol purposes. By way of illustration and not limitation, consider aplug flow tank measuring about 20 meters (width) by 100 meters (length)by 5 meters (water depth) or more. In such a tank, one might select acontrol volume which, by virtue of its location in the tank, would bereasonably representative of the process performance of the reactor. Forexample, one might choose a position centered on the longitudinalcenterline of the tank, about 24 meters from the tank upstream end and,being about 1.2 meters (wide) by 2.4 meters (long) horizontally andextending vertically throughout the depth of the wastewater in the tank.

Liquid phase mass balance involves a variety of physical, chemical andbiochemical processes that take place simultaneously. Dissolved oxygenenters and leaves the control volume as a consequence of water flowingin and out of this volume. Because water may contain oxygen (in the formof dissolved oxygen), such water entering the control volume willrepresent an input of oxygen to the control volume, and water exitingthe control volume, with whatever DO concentration is present in it,will represent an oxygen output. Other oxygen inputs may have to beconsidered, such as those due to operation of aeration devices, or, inthe simplest example, by charging pure oxygen into the control volume.Biological activity in the biomass responsible for treatment of thewastewater uses up some or all of the oxygen available in the tank. Whenestablishing a liquid phase mass balance of oxygen across a controlvolume, oxygen consumed by the biomass will no longer be present in theliquid and may therefore be considered as an oxygen output from thecontrol volume. Any other sources of oxygen output should also beconsidered when formulating this mass balance, for example oxygen outputsources such as those due to reactions that may occur, such as followingthe addition of an oxidizing agent. However, in diffused air aerationplants, oxygen input due to transfer occurring in the liquidsurface-atmosphere interface in open air aeration tanks is assumed to bea negligible fraction of the transfer taking place below the liquidsurface.

Thus, this embodiment provides a mass balance formulation in whichoxygen input and output via water flowing into and out of the controlvolume, oxygen input due to gas supply system operation, and oxygenoutput from oxygen consumption by the biomass and dissolved oxygeninventory in the control volume need to be considered.

In those cases where the total oxygen inputs to the control volume aregreater than the total oxygen outputs, a net increase of oxygen occursin the control volume, and an increase in the total oxygen inventory inthe control volume is observed. Similarly, when the total oxygen outputsare greater than the total oxygen inputs, a decrease in the total oxygeninventory will be observed.

When this mass balance is conducted on a control volume over a certainperiod of time, a given term in the mass balance relationship (whetherit be, for example, an oxygen input or an oxygen output to or from thecontrol volume) may be determined if all the remaining terms are ofknown value.

Under these circumstances, absent any other oxygen input or outputsource, biomass oxygen consumption may be determined if the oxygen inputby the gas supply system, the net oxygen input (of positive or negativevalue) due to oxygen contained in incoming and outgoing control volumewater flows and the net change (of positive or negative value) incontrol volume of dissolved oxygen inventory are known or measured.

Whenever these principles are applied to a full depth section or portionof an aeration tank located at a significant distance from the tankvertical walls, one may picture the control volume as a limited portionof the tank volume having imaginary vertical boundary surfaces that runall the way from the bottom of the tank to the liquid surface. No waterenters through the bottom (tank bottom) or top of the control volume,and all water flows enter or leave the control volume through its sideboundaries. Assuming the control volume embraces a relatively smallportion of the horizontal dimensions of the tank, whereby the dissolvedoxygen levels would be expected to vary little from one side of thevolume to the other, no significant changes in the oxygen content of thecontrol volume would be expected to occur as a result of imbalancebetween incoming and outgoing water flows, so these flows may be treatedas the same. All water flows enter the control volume from regions justoutside an imaginary boundary and all the outgoing flows depart fromregions just inside such a boundary. Because this boundary does notphysically exist, it may be assumed that liquid characteristics at bothsides of the boundary are the same. If this assumption is applied todissolved oxygen content in the liquid, it may be assumed that dissolvedoxygen at both sides of the boundary is the same. If the boundaries ofthe control volume do correspond or partially correspond with physicalboundaries, e.g., a tank wall with a small opening or an communicatingpipe, this assumption may not be applicable.

In those cases where the assumption as to imaginary boundaries isapplicable, the net oxygen input associated with liquid flowing into thecontrol volume and liquid flowing out of the control volume is of thesame value under steady state volume conditions and, thus, dissolvedoxygen concentrations in incoming and outgoing flows are the same.Therefore, the net oxygen input due to interchange of liquid between thecontrol volume and the rest of the aeration tank is zero.

Under these circumstances, the only remaining terms in the mass balanceare the oxygen input due to gas supply system oxygen transfer, theoxygen output associated with oxygen consumption by the biomass, and thenet change (of positive or negative value) in the control volumedissolved oxygen inventory.

Whenever arrangements are made so that conditions for the application ofthe described procedure are met, the amount of oxygen being consumed bythe biomass over a certain period of time may be determined from the gassupply system oxygen transfer and the net change in control volumedissolved oxygen inventory.

The net change in control volume dissolved oxygen inventory may bederived from dissolved oxygen measurements at the beginning and end ofthe time period during which a mass balance is performed and the controlvolume.

The determination of the Oxygen Transfer of the gas supply system isdone with a second mass balance on oxygen (gas phase mass balance) forthe selected control volume. This mass balance is based on the ideathat, in the absence of any other gas phase oxygen inputs and outputs,whatever amount of oxygen is depleted from the gas is equivalent to theoxygen dissolved into the liquid (oxygen transferred to the liquid).

Therefore, oxygen transfer may be determined from analysis of gasentering and gas leaving the system.

One approach to this task is to measure the oxygen entering the systemin the aeration gas and the oxygen leaving the system in the offgas bymeasuring the gas flow and oxygen content of the incoming gas and thegas flow and oxygen content in the offgas.

Another approach to this task involves assuming that both the Incomingvolumetric gas flows and outgoing volumetric offgas flows are of thesame value as a consequence of no net changes of gas volume in thesystem (gas volume in the system remains constant with time).

A suitable way of determining the amount of oxygen present in both theincoming and the outgoing gas streams could be to compare the oxygenpresent in each gas stream with other components present in each gasstream that remain constant through the process. By way of example, ifthe aeration gas used contains a certain portion of gas A that is nottransferred to the liquid and does not react with the tank contents(inert), then both the incoming gas stream and the outgoing gas streamswould show the same content of gas A. Gas A is conserved during theprocess.

In order to do so, it may be necessary to measure the carbon dioxide andwater vapor content of the incoming and outgoing gas streams.

Oxygen depletion in the gas phase or oxygen transfer to the liquid phasemay then be expressed as a percentage reduction in oxygen content in thegas stream by comparing the difference between the molar ratios ofoxygen to inerts in the incoming and outgoing streams to the molar ratioof oxygen to inerts in the incoming stream.

Whenever this approach is followed, the percentage oxygen transferdetermined (Oxygen Transfer Efficiency) may be combined with oxygeninput rate data to determine Oxygen Transfer Rate. By way of example,the mass of oxygen transferred may be determined from the percentageoxygen transfer observed and the mass flow of oxygen introduced into thesystem. In many instances it may be convenient to express mass balanceequations in terms of rate units (oxygen transfer rate, oxygen uptakerate, and net oxygen inventory change rate) instead of mass units.

Exemplary Variables Involved

-   OI_(to)=Oxygen inventory in a control volume at the beginning of the    time period during which a liquid phase oxygen mass balance is    performed-   OI_(tmb)=Oxygen inventory in a control volume at the end of the time    period during which a liquid phase oxygen mass balance is performed-   OTE=Oxygen Transfer Efficiency-   OTR=Oxygen Transfer Rate-   OUR=Mixed Liquor Oxygen Uptake Rate-   O_(2conc)=Concentration of Oxygen in Oxygen containing gas-   Q=Oxygen containing gas volumetric flow into the control volume-   t_(mb)=Time constant set to establish the period of time during    which a liquid phase oxygen mass balance is performed-   V=Volume, may relate to the complete tank or part of it    Values associated with these variables within the control system may    be stored in or developed by the control system with the aid of data    within the system or acquired from external sources.

Determination of Oxygen Transfer Characteristics

The information gathered during the calculations conducted to determineOUR may also be used to assess the oxygen transfer characteristics ofthe system studied if appropriate data are available. In order to do so,some relationship between the Oxygen Transfer of a gas supply system inprocess conditions and the Oxygen Transfer of the same system underknown conditions may be used.

In the aeration industry, oxygen transfer of gas supply systems anddevices is commonly expressed in relation to a set of referenceoperating conditions to allow comparison of different equipment underequivalent conditions. This is due to the fact that gas supply systemoxygen transfer depends on factors such as ambient conditions(barometric pressure and water temperature amongst others), watercharacteristics (composition, etc.) and dissolved oxygen concentrationin the aeration basin that would make data from different aerationdevices very hard to compare unless operating under similar (if notexactly the same) conditions.

When comparing oxygen transfer of a gas supply system operating underprocess conditions with oxygen transfer of the same system underreference conditions (Standard Conditions), a number of correctionfactors are preferably introduced to account for the different effect ofdifferent operating conditions on system performance.

In addition, tests at reference conditions are usually conducted on newgas supply systems, so in those cases where oxygen transfer of a gassupply system may be influenced by gas supply system condition (newversus used systems), another correction factor can be introduced toaccount for the effect of gas supply system condition on oxygentransfer.

Correction factors for ambient conditions such as water temperature,barometric pressure and water temperature have been documented in theliterature and widely accepted and extensively used in the past.

However, due to the difficulty of establishing a relationship betweenwastewater characteristics and composition and its effect on oxygentransfer, no widely accepted correction factors have been establishedfor the determination of oxygen transfer of a gas supply system inwastewater compared to its performance under reference conditions, e.g.,in potable water.

If ambient condition correction factors are used in combination with thevalues of the parameters involved in the above-mentioned corrections,some of which may require measuring and others of which may be assumed,a relationship between the oxygen transfer of the gas supply system asmeasured in process conditions and the oxygen transfer of the same gassupply system under standard conditions could be developed in which allterms in the relationship would be known (measured or calculated) exceptthe effects of (a) wastewater characteristics and (b) gas supply systemcondition. Therefore, even if the individual values of these twoparameters were not identified, their combined effect could bedetermined. Once this effect has been determined (apparent alpha), arelationship between standard conditions gas supply system oxygentransfer and process conditions gas supply system oxygen transfer, whereall correction factors are known or established, could be developed andbe useful in establishing gas supply system performance control values.

Determination of the oxygen transfer characteristics of the gas supplysystem and mixed liquor of the process involves measuring both thewastewater temperature in the control volume and the Dissolved Oxygen inthe mixed liquor. Although C*_(∞f) can be calculated from measuredvalues such as Barometric Pressure, Wastewater Temperature and SalinityCorrection factor β, its small variation suggests the possibility ofusing built in relationships, meaning that the control system couldoperate successfully on the basis of fixed values for C*_(∞f) stored orintroduced temporarily into the control system. Thus, a control systembuilt according to this embodiment of this invention will include one ormore DO (dissolved oxygen) sensors and one or more temperature sensors,as will be discussed below in conjunction with the accompanyingdrawings.

Exemplary Variables Involved

-   α=Effect of wastewater characteristics on gas supply system ability    to transfer oxygen into wastewater-   αF=Apparent alpha value, combined effect of wastewater    characteristics (a), and gas supply system condition (F), on gas    supply system ability to transfer oxygen to wastewater-   β=Correction factor for the effect of salinity on dissolved oxygen    saturation concentration-   C*_(∞20)=Dissolved oxygen saturation concentration at 20° C., 1 atm-   C*_(∞f)=Dissolved oxygen saturation concentration in field    conditions-   DO=Mixed liquor dissolved oxygen-   K_(La)=Apparent volumetric mass transfer coefficient-   F=Effect of gas supply system condition (often associated to    diffuser fouling/aging) on gas supply system ability to transfer    oxygen-   OTR=Oxygen Transfer Rate-   P=Barometric Pressure-   SOTR=Oxygen Transfer Rate at Standard Conditions (20° C., 1 atm, 0    DO, clean water)-   θ′=Correction factor for effect of wastewater temperature on gas    supply system ability to transfer oxygen to wastewater,    θ′=θ^((T-20)), where T is wastewater temperature-   θ=Arrhenius coefficient for wastewater temperature correction factor    to account for wastewater temperature effect on oxygen transfer-   T=Wastewater Temperature    Values associated with these variables within the control system may    be stored in or developed by the control system with the aid of data    within the system or acquired from external sources.

Establishment of Oxygen Requirements

As previously mentioned, in the present embodiment of the invention, theapproach taken in the control system for determining the oxygenrequirements of the biological process the system at any point in timeincludes satisfying the oxygen requirements of the biological treatmentprocess and maintaining a preestablished or target DO concentration atone or more selected locations in the process mixed liquor.

Although determination of the oxygen requirement of the biologicaltreatment process has been discussed above, the present embodiment alsodetermines the oxygen required to keep the process at a preestablishedDO concentration as a function of the actual process conditions withrespect to the preestablished conditions (DO target level). Wheneveractual process conditions match the preestablished target conditions,both objectives of the control strategy are met. The process is takingup oxygen at the rate at which it is being supplied and operates at thedesired dissolved oxygen level.

However, if the actual process conditions differ from the targetconditions, a difference between the actual DO concentration at theselected control point in the mixed liquor and the target DOconcentration at that same control point is observed. This may happenbecause the DO in the aeration basin is higher than the target value orlower than the target value. In both cases, DO control values should bedeveloped and corrective actions implemented to return DO levels totarget DO levels. If only the higher or lower amount of oxygen requiredby a change in biomass consumption of oxygen were supplied, thedifference observed as to the DO level in the process versus the DOtarget level would remain present. An additional amount of oxygen shouldbe supplied when process DO is lower than target DO, and a lesser totaloxygen supply than that required by biomass consumption should besupplied when process DO is higher than target DO.

The needed increment of increased or decreased oxygen supply, above orbelow that required to meet current biomass requirements may bedetermined by establishing a relationship between observed processconditions and target process conditions. This may be done byconsidering the dissolved oxygen inventory in a selected control volumearound the target DO control location. More particularly, the controlsystem determines how much dissolved oxygen would be present in themixed liquor if the target DO were achieved and how much DO is actuallypresent in the same volume. The difference between these two quantities,positive or negative, is then added or subtracted by the control systemfrom the amount of oxygen required for biomass consumption.

Because oxygen requirements are usually expressed as rates, the resultof this DO inventory, i.e., the total mass of oxygen to be added orsubtracted from biomass requirements, will usually be converted into anoxygen supply rate required to return DO to the target value over aselected time period. Introduction of a time parameter establishes thespeed at which the DO level will be returned to the target value.

Target DO refers to a selected level of DO which the operator wishes tomaintain at a selected control location and t_(c) refers to a timeconstant, the period of time in which it is desired to return DO to thetarget DO level. ΔDO refers to the difference between the target DO andthe mixed liquor DO (dissolved oxygen content of the wastewater) for theselected control location.

Exemplary Variables Involved

-   DO=Mixed liquor dissolved oxygen-   ΔDO=Difference between target dissolved oxygen concentration and    actual dissolved oxygen concentration at a selected location-   DO_(target)=Target DO concentration for a selected location-   t_(c)=Time constant to establish the time set to correct actual DO    to Target DO-   V=Volume, may relate to the complete tank or part of it    Values associated with these variables within the control system may    be stored in or developed by the control system with the aid of data    within the system or acquired from external sources.

Determination of Gas Supply System Required Operating Conditions

Once the oxygen requirements needed to meet established goals isdetermined, the relationship between gas supply system oxygen transferin process conditions and gas supply system oxygen transfer in standardconditions, developed as described above, is used by the control systemto determine the standard conditions oxygen supply required by theprocess.

Data available on standard condition performance of the gas supplysystem, which data may be stored in or developed by the control systemwith the aid of data within the system or acquired from externalsources, may then be useful in determining gas supply system operatingconditions and performance control values required to achieve thedesired oxygen supply.

Exemplary Variables Involved

The following is a key to certain expressions used in the abovedescription and in the measurement and calculation of process variables:

-   α=Effect of wastewater characteristics on gas supply system ability    to transfer oxygen into wastewater-   αF=Apparent alpha value, combined effect of wastewater    characteristics (α), and gas supply system condition (F), on gas    supply system ability to transfer oxygen to wastewater-   β=Correction factor for the effect of salinity on dissolved oxygen    saturation concentration-   C*_(α20)=Dissolved oxygen saturation concentration at 20° C., 1 atm-   C*_(αf)=Dissolved oxygen saturation concentration in field    conditions-   DO=Mixed liquor dissolved oxygen-   DO_(target)=Target DO concentration for a selected location-   F=Effect of gas supply system condition (often associated to    diffuser fouling/aging) on gas supply system ability to transfer    oxygen-   OUR=Mixed Liquor Oxygen Uptake Rate-   Q=Oxygen containing gas volumetric flow into the control volume-   ROTR=Total Required Oxygen Transfer Rate under process conditions-   SOTR=Oxygen Transfer Rate at Standard Conditions (20° C., 1 atm, 0    DO, clean water)-   θ′=Correction factor for effect of wastewater temperature on gas    supply system ability to transfer oxygen to wastewater,    θ′=θ^((T-20)), where T is wastewater temperature-   θ=Arrhenius coefficient for wastewater temperature correction factor    to account for wastewater temperature effect on oxygen transfer-   T=Wastewater Temperature    Values associated with these variables within the control system may    be stored in or developed by the control system with the aid of data    within the system or acquired from external sources.

Adjustment of Gas Supply System Operating Conditions

All of the steps described in previous sections cover the differentprocedures and methods used to establish the aeration operatingconditions required to achieve the control goals established.

Once individual SOTR values applicable to one or more control volumesand/or complete tanks are established, the control system uses thisinformation to adjust gas supply system parameters and devices, usingthe correlation between gas supply system performance at processconditions and at standard conditions. In most cases, gas supply systemoperating conditions can be defined as a function of individual/totalgas flows to each control zone/complete tank.

DEFINITIONS

“Adjust” or “adjustment” refers to: modifying data from a measuringdevice or control signals from a controller, including for example achange in magnitude and/or conversion to a different form. These termsalso refer to altering one or more biological process parameters andaltering one or more conditions of some part of the biological processequipment and/or of the control system. Usually, such altering is inresponse to some indication of need, which may be a changing need foroxygen-containing gas, such as the need for gas consumed in thebiological process, and/or the need for gas to change a DO level and/orthe need for gas occasioned by changes in gas supply system performance.Such altering may occur on a continuous or intermittent basis. In someinstances, alteration can occur in such a way that the full amount ofcorrective action required to meet one or more needs occurs immediately,when the control system senses the need. In other instances, alterationcan occur over a period of time, in increments. For incrementalalterations, it is not possible to state for all situations the absoluteminimum proportion of the corrective action that must be applied in thefirst and subsequent increments. Biological treatment plants can varywidely in their time of response to corrective actions. When theinvention is embodied in ways that involve continuing but incrementalalteration, system wait times can vary widely. However, alterations canoccur in increments representing a small proportion of the totalcorrective action desired when wait times are short and/or plantresponse time is long. Conversely, larger increments may be requiredwhen wait times are long and/or plant response times are short. Armedwith this understanding and their experience with plant operations,persons skilled in the art can determine, without undue experimentation,what proportion of the total corrective action should be applied in therespective increments, so that there will be a sufficient amount ofcorrective action per increment to prevent changing needs fromfrequently or seriously out-running the control system.

“Aerobic biological process” means any of a variety of biologicalprocesses, one or more portions of which are supported, at least in partby the introduction of oxygen containing gas into wastewater in order tocreate an aerobic environment. Prominent examples of these processesexist in a wide variety of continuous and discontinuous configurationsof the activated sludge process involving a variety of flow regimes.Examples include plug flow, complete mix and step feed aeration.Submerged aerated filters and other batch processes are contemplated inwhich the wastewater is aerated for all or a portion of the operationcycle for each batch.

“Amount”, as applied to any given tangible or intangible thing,including without limitation materials, data and signals, refers to aquantity of that thing or a quantity relationship between that thing andanother tangible or intangible thing. Such quantity or relationship maybe expressed in any unit or units or without units. For example, anabsolute quantity may be expressed in units of, e.g., mass or volume. Arelative quantity may be expressed, e.g., as units of the given thingper unit time (rate) or per unit volume or mass of another thing, or asa ratio between different things which, e.g., are expressed in the samekinds of units, so that the nature of the units may be ignored.

“Approximate” means that there is a degree of correlation between valueswhich, whether perfect or imperfect, is sufficient to be useful incontrolling a wastewater aeration process in accordance with theinvention.

“Biological process” means any wastewater treatment process which, atleast in part, involves the metabolization by bacterial action of wastematerial dissolved and/or suspended in wastewater, that encompasses,among others, one or a combination of aerobic, anoxic, and anaerobicsteps or processes.

“Composition”, as applied to a gas, refers to the identities of at leasta portion of the gases in a mixture of two or more different gases, orto the relative amounts of two or more gases in such a mixture, or tothe amount of a single gas in such a mixture.

“Connected with” means having a tangible or intangible operationalconnection, whether direct or indirect, including such tangible forms ofconnection as dedicated wires, electric power lines and wiring systems,intranet or internet connections, telephone lines, fiber-optic cables,connections on circuit boards and pneumatic signaling lines, and suchintangible forms of connections as radio waves, laser and other lightbeams, and sound waves, by which control system resources such as data,control signals or outputs, control inputs and code may pass betweencooperating components of the control system, e.g., measuring devices,controllers and flow regulating devices, whether such components arelocated close to or distant from one another.

“Consumption of oxygen . . . in the biological process” refers to oxygenthat is consumed, e.g., by bacteria or other means, in removing from thewastewater and/or in otherwise acceptably altering, e.g., bymetabolization and/or by other mechanisms, carbonaceous, and/ornitrogenous and/or other forms of waste; this language is intended todistinguish process oxygen needs from deficiencies and excesses in thesupply of oxygen to the wastewater which manifest themselves asdecreases and increases in the DO level of the wastewater.

“Continuing”, for example as in the exercise of continuing control orthe taking of continuing measurements, refers to actions taken on acontinuous basis or on an intermittent but repetitive, including aperiodic or irregularly repeating basis.

A “controller” is any device which is or includes one or more logicdevices, and is able, whether alone or in combination with one or moreother devices, to interpret values correlative with one or moreparameters of the biological process and to establish control values.

The controller may for example be at least in part, including wholly,one or more mechanical devices and/or one or more electrical and/orelectronic devices. Thus, the logic of the controller may for examplereside at least in part in one or more mechanical relationships inmechanical devices, electrical relationships in electrical and/orelectronic devices, and/or in any combination of the foregoing.

The controller preferably includes or at least has access to appropriatesoftware or code to interpret data on process conditions gathered frommeasurement apparatus and establish the control values. In a preferredembodiment, the logic resides at least in part, which may includewholly, in one or more elements of code temporarily present or stored inone or more co- or remotely-located programmed or programmable devices.

Controllers used in the invention may be specialized units of limitedbut sufficient computing capacity, or may be a general- orspecial-purpose computer or computers of considerable computingcapacity. The controller is preferably capable of executing basiccontrol instructions (e.g., Boolean logic and four function math) suchas those commonly available through (but not limited to) computer orpersonal computer (PC) based control platforms, programmable logiccontroller (PLC) based control platforms, or distributed control systems(DCS) based control platforms. Proportional, proportional-integral (PI)and proportional-integral-derivative (PID) controllers may be used. See,e.g., “Process Instruments and Controls Handbook”, 3d Ed., McGraw Hill.

The controllers may also include memory devices, as well as comparators,other devices and/or code that adjust, refine, correct, condition orotherwise assist by performing auxiliary functions, such as tuning thecontrol system and/or processing data, control values and controlsignals. Thus, adaptive (self- or auto-tuning) or non-adaptivecontrollers may be used.

In effect, the controller defines, for the varying amounts of biologicalconsumption of oxygen that occur in the process, control values, orcomponents of control values, that change in response to, whileremaining correlative with, such varying amounts of oxygen consumption.Put differently, the controller generates varying control values whichhave, or which respectively include at least one component that has, ona continuing basis, an at least approximate quantitative relationshipwith the varying amounts of oxygen consumed by the biological process.

Control values generated by the controller, with or without intermediateadjustment, are useful for acting on the process, or on items such asvalves or other control elements associated with it, to alter ormaintain operation of the process in a way that generally limits orminimizes deviation of one or more process variables from desiredperformance, for example from established set points. Control values ofmore than one type, e.g., respectively corresponding with more than oneprocess need, may be combined within the controller, e.g., to generate asingle control signal involving plural components. Optionally, controlvalues that respectively represent different process needs may begenerated but kept separate within the controller and used to issueseparate control signals to different control elements.

Correlative with”, as applied to a relationship between first and secondvalues, means that, regardless of whether or not they are numericallyequal or precisely related, there is at least an approximatequantitative relationship between them, a sufficient degree ofrelatedness so that they or at least one of them can serve as apractical basis for control over the process. The magnitude of one ormore of the values may be affected by its inclusion of one or moreparameters, usually small enough to be ignored, that are not part of therelationship on which the correlation is based. In embodiments of theinvention in which a first value is correlative with but not numericallysimilar to a second value, the first value may be functionally relatedto the second in such a way that the first may be used as an at leastapproximate indicator of the other. Any useful functional or other typeof relationship between the values will suffice. The relationship maytake any useful form. For example, one value may be directlyproportional to the second. Or the first may be related to the second bya fixed or variable difference. Or the first may be related to thesecond through an equation or table of values. Values of all kinds areincluded, for example Oxygen Transfer Rate vs. Gas Flow, and OxygenTransfer Efficiency vs. Gas Flow. In the case of control values,“correlative with” preferably refers to a relation between (a) anapplied control value applied by the system in relation to a particularprocess control needs, e.g., process oxygen needs, DO level controlneeds, performance control needs or a combination of process controlneeds and (b) a reference control value which would adjust operation ofthe biological process in a way that would precisely satisfy theparticular need or needs; in such relation, the applied control value,whether applied in one or a plurality of increments, approximates thereference control value. The adequacy of this approximation will beexpressed in conventional usage as a percentage difference between thecontrol value and the reference value, said difference being plus orminus 20%, more preferably plus or minus 10%, still more preferably plusor minus 5%, and most preferably plus or minus 3%. Notwithstanding thisconventional usage, as the reference value nears the upper and lowerbounds of the useable range it may be more convenient or more accurateto express the adequacy of this approximation as a finite difference,e.g. plus or minus 0.10 ppm or plus or minus 25 cubic meters per hour.

“DO control values” or “dissolved oxygen control values” refer tomeasured and calculated parameters correlative with the amount of oxygenrequired to move a DO level (including positive oxygen or zero oxygenconditions) observed in the process to or toward a target DO level.

“Gas collection member” means a device comprising a confined chamber forreceiving from wastewater and substantially isolating from theatmosphere at least a portion of the gas bubbles that have been releasedinto wastewater by a gas supply system and have traveled upward in thewastewater for at least a portion of its depth but have not beendissolved in the wastewater. A typical but non-limiting example would bea hood, rectangular in plan view and triangular in transversecross-section, having an open bottom; except for inlets and outletsassociated with its control function, it is otherwise gas tight and isequipped throughout the periphery of its lower edges with floats tosupport it at the surface of wastewater. Gas collection members need nothowever be located at the wastewater surface, since they can performtheir receiving and isolating functions if positioned beneath thesurface or if positioned above the surface and provided with dependentskirts extending, throughout their periphery, in a direction down towardand preferably to a position beneath the surface.

“Gas supply system” includes any bubble-forming device or devices ofwidely varying type, shape and size that is/are suitable fortransferring the oxygen of an oxygen containing gas to wastewater in thecontext of a biological treatment process, for example area release finebubble diffusers, draft tube aerators, mechanical aerators, brushaerators and coarse bubble diffusers, along with the necessary accessoryequipment to support the operation of the bubble forming device ordevices and deliver the gas thereto, including gas supply conduits,manifolds, support stands, downcomers, yard piping, valves, filters,positive displacement compressors, turbo-compressors, or centrifugalblowers and related compressor/blower control and gas flow controldevices. Illustrative area release fine bubble diffusers include thosein the form of tubes, disks, domes and sheets, whether of elastomeric,ceramic or fibrous material. Examples of coarse bubble diffusers includehood, nozzle, orifice, valve and shear devices.

“Indicative” refers to the quality of indicating a given valuenumerically equally or, if not numerically equally, at least through afunctional or other relationship, such indication being a precise valueso far as can be determined by observation or calculation from the dataavailable in the system or, if not such a precise value, deviating fromthe precise value by an amount insufficient, taking into account theintended use of the indication, to destroy its usefulness for effectingcontrol over the biological process. In a preferred embodiment, theindication is within +/−20%, or +/−10%, or +/−5% or +/−3% of saidprecise value.

“In response to” refers to direct and indirect stimulation of an actionor condition by another action or condition; for example a controlelement acts in response to a control signal when such action is adirect or indirect result of the control signal, whether the signal isreceived directly or indirectly from a controller with or withoutmodification or conversion to a different form.

“Mixed Liquor” refers to the contents of a tank comprising at leastwastewater and biomass.

“Oxygen-containing gas” includes any gas, including mixtures of gaseswith or without entrained or dissolved vapors, for example air, oxygen,ozone, any other gas and mixtures of any of these, that is suitable tosupport an aerobic biological process or process step for the treatmentof wastewater, such as a suspended growth aeration process andpreferably a process that includes one or more activated sludgeprocessing steps.

“Oxygen Uptake Rate” (OUR) refers to the time rate of consumption ofoxygen in the wastewater, and includes components such as biomass oxygenconsumption, other forms of oxygen consumption, chemical reactions, andother factors.

“Performance parameters” refer to measured, calculated, or predeterminedvalues that are correlative with changes in performance or efficiency ofany device or process in the system.

“Provide” or “Providing” means making available in any manner for use inthe control system for any useful period. For example, as applied tocode or data, the definition includes making same available from withinor without the system, from a source at or remote from the site at whichthe biological process is conducted, by generating same in the system,and/or by manually inputting same into the system, and/or by storage ofsame in the system, whether or not the storage location is within thesystem and whether or not storage is brief or long term, with or withoutbeing updated from time to time.

“Repetitive” means repeated at a time interval of any length which isuseful in effecting control of an aeration operation in the context ofthe invention, for example, at intervals of up to about 8 hours, morepreferably up to about 1 hour and still more preferably up to about 5minutes. These intervals may be as short as a very small fraction of asecond, e.g., about 0.01 second or more, preferably about 10 seconds ormore and more preferably about 30 seconds or more.

“Requirements control values” refers to measured and calculatedparameters correlative with the oxygen required to satisfy the usage ofoxygen in the biological process. These may include but are not limitedto all factors related to oxygen uptake rate (OUR) in both steady stateand non-steady state conditions.

“Suspended growth aeration process” means an aerobic biological processin which oxygen containing gas usually assists in mixing the wastewater,and still more preferably, assists in maintaining the bacteria insuspension.

“Tank” refers to one or more suitable natural and/or man-made waterimpounds which may be of widely varying type, shape and size. Thus, thetank or tanks may be earthen- or plastic-lined, but are preferably ofsteel or concrete and are of any suitable shape when viewed in plan viewor vertical section. For example, the tanks may have a circular,annular, oval, square or elongated rectangular shape in plan view. Theterm tank also applies to a section of a tank that has been segregatedfrom one or more other portions of that tank by a baffle and/or otherform of length divider so that the segregated section respondssubstantially independently of the other section or sections to controlinputs. Preferred are tanks in which their dimensions in the directionof wastewater flow, whether in a straight line or not (L), are greaterthan their dimensions perpendicular to such direction (W), in which L/Wmay, e.g., be greater than 3, 5, 10 or 15, such as tanks of annular orelongated rectangular shape. Preferably, at least the aerobic portionsof the tanks will be equipped with any suitable gas supply system.

“Values” are representations of (a) quantities, expressed in anysuitable unit or combination of units, such as units of mass, volume,pressure, time, electrical potential, resistance or other units, orexpressed as unitless numbers, or of (b) conditions, e.g., “on”, “off”,“above”, “below”, “equal to” and others. The results of measurements areusually expressed as values.

“Wastewater” refers to the wastewater undergoing treatment at any stagein a biological process, encompassing among others raw wastewater,wastewater after preliminary treatment, mixed liquor and other mixturesof wastewater and biomass.

1. A method of exercising continuing control over an oxygen-consumingbiological wastewater treatment process in which the need for oxygenrepeatedly increases and decreases and which is conducted in a pluralityof wastewater treatment plant processing tanks in cooperation with a gassupply system to supply oxygen-containing gas bubbles to, and dissolveoxygen in, the wastewater in at least one of said plant processing tanksand a control system comprising at least one flow control element tosupply an increasing and decreasing flow of oxygen-containing gasthrough the gas supply system into the wastewater in the at least oneplant processing tank, at least one gas collection member and gasdetector to provide off-gas data correlative with changing amounts ofone or more gases in offgas from the wastewater, a controller to processthe offgas data and cause the flow control element to increase anddecrease the flow of oxygen-containing gas into the wastewater in saidtank or tanks, and wherein the control system is programmed to tend tomaintain a positive DO level in at least a portion of the at least onetank which method comprises: providing in the control system DO(dissolved oxygen) data correlative with varying DO levels in thewastewater and/or performance data correlative with varying ability ofthe gas supply system to dissolve oxygen in the wastewater, generatingcontrol values in the control system derived at least in part from (a)the offgas data and (b) the DO data and/or performance data and usingsuch control values to generate control signals to cause the at leastone flow control element to cause varying flows of oxygen-containing gasthrough the gas supply system and into the at least one processing tankthat are correlative with the varying consumption of oxygen by thebiological process adjusted to cause wastewater DO levels to movetoward, return to or be maintained at a target value and/or compensatefor the varying ability of the gas supply system to dissolve oxygen inthe wastewater.
 2. A method according to claim 1 comprising generatingcontrol values in the control system derived at least in part from theoffgas data, DO data and performance data and using such values togenerate control signals to cause the at least one flow control elementto provide flows of oxygen-containing gas into the at least one plantprocessing tank reflecting process oxygen needs adjusted to (a) causewastewater DO levels to move toward, return to or be maintained at atarget value and (b) compensate for the varying ability of the gassupply system to dissolve oxygen in the wastewater.
 3. Method accordingto claim 1 wherein the biological process comprises suspended growthaeration which includes biological metabolization of suspended and/ordissolved waste material present in the wastewater.
 4. Method accordingto claim 1 wherein the controller contains or has access to tables ofdata, with the aid of which it defines said control values.
 5. Methodaccording to claim 1 wherein the control system operates as a feedforward controller wherein control outputs are generated, at least inpart, based on requirements control values and performance controlvalues.
 6. Method according to claim 1 wherein said control valuescomprise plural control value components combined within the controllerto generate one or more control signals.
 7. Method according to claim 6wherein said control values comprise, as component parts thereof,requirements control values combined with DO rate of change values andDO control values.
 8. Method according to claim 1 comprising providinggas supply system operational performance data in the control system. 9.Method according to claim 1 comprising providing gas supply systemperformance standard data in the control system.
 10. Method according toclaim 9 comprising providing gas supply system relative systemperformance data in the control system that is derived at least in partwith performance standard data.
 11. Method according to claim 1 whereinDO levels in the wastewater differ positively and/or negatively from atarget DO value and the system generates DO control values and controlsignals which are sufficient, when applied in conjunction withrequirements control values generated by the system, to at leastpartially offset deviations of the DO levels from the target DO value.12. Apparatus for exercising continuing control over an oxygen-consumingbiological wastewater treatment process in which the need for oxygenrepeatedly increases and decreases and which is conducted in a pluralityof wastewater treatment plant processing tanks in cooperation with a gassupply system to supply oxygen-containing gas bubbles to, and dissolveoxygen in, the wastewater in at least one plant processing tank, and acontrol system comprising at least one flow control element to supply anincreasing and decreasing flow of oxygen-containing gas through the gassupply system into the wastewater in the at least one plant processingtank and at least one gas collection member and gas detector to provideoff-gas data correlative with changing amounts of one or more gases inoffgas from the wastewater, a controller to process the off-gas data andcause the flow control element to increase and decrease the flow ofoxygen-containing gas into the wastewater in said tank or tanks, andwherein the control system is programmed to tend to maintain a positiveDO level in at least a portion of the at least one tank characterized inthat the apparatus comprises at least one DO (dissolved oxygen) detectorto provide, in the control system, DO data reflecting DO levels in thewastewater and the controller contains or has access to code which, withthe aid of the offgas data and DO data, the controller defines varyingcontrol values comprising separate or combined requirements controlvalues correlative with the repeatedly fluctuating need foroxygen-containing gas flow to support the biological process and DOcontrol values that are correlative with such varying positive ornegative adjustments of oxygen-containing gas flow sufficient to causethe wastewater DO levels to move toward, return to or be maintained at atarget value, the at least one flow control element is connected withthe controller to receive and act in response to control signals in thecontrol system based at least in part on said control values to supplyan increasing and decreasing flow of oxygen-containing gas through thegas supply system into the wastewater in the at least one plantprocessing tank.
 13. Apparatus according to claim 12 wherein thecontroller contains or has access to code which, with the aid ofperformance data, the controller defines performance values that arecorrelative with additional oxygen-containing gas flow adjustmentsneeded to compensate for varying ability of the gas supply system todissolve oxygen in the wastewater.
 14. Apparatus according to claim 12comprising a tank having a wastewater inlet and an outlet, and thecontrol system includes DO measuring devices at first and secondlocations in the tank.
 15. Apparatus according to claim 12 wherein theat least one tank is divided into at least two sections by a baffleand/or other form of length divider, at least one of said sectionshaving upstream and downstream halves.
 16. Apparatus according to claim12 wherein the gas collection member is positioned in an upstream halfof a tank or tank section to receive offgas representing gas frombubbles that have not been dissolved in the wastewater.
 17. Apparatusaccording to claim 12 wherein the gas collection member is positioned inan upstream half of a tank or tank section to receive offgasrepresenting gas from bubbles that have not been dissolved in thewastewater.
 18. Apparatus according to claim 12 wherein the controlsystem includes at least two DO probes respectively positioned inupstream and downstream halves of a tank or tank section for gatheringdata with respect to DO levels.
 19. Apparatus according to claim 12wherein the control system includes at least two DO probes respectivelypositioned in upstream and downstream halves of a tank or tank sectionfor gathering data with respect to DO levels.
 20. Apparatus according toclaim 12 comprising a tank or tank section having an upstream end, andat least portions of the gas collection member and of a DO probe arepositioned within an interval of about the first 20% of tank length,measured from the upstream end.